Photoelectrocatalytic oxidation of methane over three-dimensional ZnO/CdS/NiFe layered double hydroxide
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摘要: 將甲烷以低能耗的方式直接轉化為甲醇等高附加值的化學品一直是可持續化工產業的重要目標和重大挑戰。本文制備了三維（3D）ZnO/CdS/NiFe層狀雙金屬氫氧化物（LDH）核/殼/分層納米線陣列（NWAs）結構材料并將其用于室溫、模擬陽光照射下甲烷的光電催化氧化。結果表明3D ZnO/CdS/NiFe-LDH具有優異的光電化學性能及催化活性，甲烷氣氛下的光電流密度達到了6.57 mA·cm?2（0.9 V vs RHE），其催化甲烷生成甲醇及甲酸產量分別是純ZnO的5.0和6.3倍，兩種主要產物的總法拉第效率達到54.87%。CdS 納米顆粒（NPs）的沉積顯著提升了復合物對可見光的吸收，促進了光生載流子的分離。而具有三維多孔結構的NiFe-LDH納米片的引入改善了甲烷氧化表面反應動力學，起到了優異的助催化作用；并且有效抑制了O2?-的產生，防止O2?-進一步將甲醇及甲酸氧化為CO2，提高了甲醇及甲酸的選擇性。最后，提出了三維ZnO/CdS/NiFe-LDH復合材料光電催化甲烷轉化為甲醇及甲酸的機理，為甲烷低能耗轉化為高價值化學品提供了新思路。Abstract: The direct conversion of methane into methanol and other high value-added chemicals with low-energy consumption has always been an important goal and a major challenge for the sustainable chemical industry. In this paper, a three-dimensional (3D) ZnO/CdS/NiFe layered double hydroxide (LDH) shell/core/hierarchical nanowire arrays (NWAs) structure material was fabricated and utilized for photoelectrocatalytic oxidation of methane at room temperature under simulated sunlight. Results show that the ZnO/CdS/NiFe-LDH photoanode exhibites excellent photoelectrochemical performance and catalytic activity. The photocurrent density under the methane atmosphere reached 6.57 mA·cm?2 at 0.9 V (vs RHE). Yields of methane oxidation products, which mainly are methanol (CH3OH) and formic acid (HCOOH), catalyzed by the synthesized ZnO/CdS/NiFe-LDH composite are 5.0 and 6.3 times those of pure ZnO, respectively. The total Faraday efficiency of the two main products reach 54.87%. The deposition of CdS nanoparticles (NPs) significantly facilitates the absorption of visible light and promotes the separation of photo-generated carriers. The introduction of NiFe-LDH nanosheets with a three-dimensional porous structure improves the surface reaction kinetics of methane oxidation, acting as an excellent co-catalyst. It also effectively inhibites the production of O2?-, preventing O2?- from further oxidizing methanol and formic acid into CO2, which improves the selectivity of methanol and formic acid. Finally, this paper proposed a mechanism of the photoelectrocatalytic oxidation of methane to methanol and formic acid over 3D ZnO/CdS/NiFe-LDH composite material, which provides a new idea for the conversion of methane into high-value chemicals with low-energy consumption.
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Key words:
- photoelectrocatalysis /
- methane /
- methanol /
- ZnO /
- NiFe-LDH
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圖 2 形貌及元素組成。（a）ZnO的掃描電鏡圖；（b）ZnO/CdS的掃描電鏡圖；ZnO/CdS/NiFe-LDH的掃描電鏡圖（c）、X射線能譜圖（d）、透射電鏡圖（e）及高分辨透射電鏡圖（f）
Figure 2. Morphology and element composition: (a) SEM images of ZnO; (b) SEM images of ZnO/CdS; SEM images (c), EDS spectra (d), TEM image (e)，and HR-TEM image (f) of the ZnO/CdS/NiFe-LDH sample
圖 3 ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH的X射線衍射圖
Figure 3. XRD patterns of ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH photoanodes
圖 4 ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH的紫外?可見光吸收譜
Figure 4. UV–vis absorption spectra of ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH
圖 5 光陽極的線性伏安掃描曲線。（a）ZnO；（b）ZnO/CdS；（c）ZnO/CdS/NiFe-LDH；（d）在持續通入甲烷且光照條件下對比圖
Figure 5. LSV: (a) ZnO; (b) ZnO/CdS; (c)ZnO/CdS/NiFe-LDH photoanodes; (d) comparison in the presence of CH4 under illumination
圖 6 ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH光陽極在持續通入甲烷下的（a）1.1 V（vs RHE）下的計時電流（J?t）曲線；（b）光照條件下與黑暗條件下的電化學阻抗譜；（c）1.1 V（vs RHE）下的電流?時間穩定性曲線
Figure 6. (a) Chronoamperometric J?t curves collected at 1.1 V (vs RHE) under chopped illumination conditions; (b) EIS plots under dark and light illumination; (c) current–time curves for stability measurement collected at 1.1 V (vs RHE) of ZnO, ZnO/CdS, ZnO/CdS/NiFe-LDH photoanodes saturated with methane
圖 7 外加電壓為1.1 V(vs RHE)，光照強度為AM 1.5G、100 mA·cm?2，電解液為0.5 mol·L?1 Na2SO4水溶液條件下ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH光電催化氧化甲烷轉化為甲醇（a）甲酸（b）產量隨時間變化曲線圖以及2.5 h內生成甲醇及甲酸的法拉第效率（c）
Figure 7. Yields of CH3OH (a) and HCOOH (b) in the photoelectrocatalytic oxidation of CH4 with ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH catalysts, with a potential of 1.1 V (vs RHE) under simulated sunlight illumination (AM 1.5G, 100 mA·cm?2), and the electrolyte is 0.5 mol·L-1 Na2SO4 aqueous solution; (c) Faradaic efficiencies of CH3OH and HCOOH for ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH for a 2.5-h operation
圖 8 ZnO/CdS/NiFe-LDH光電催化氧化甲烷過程電荷轉移原理圖
Figure 8. Schematic diagram of the charge transfer in ZnO/CdS/NiFe-LDH for photoelectrocatalytic oxidation of methane
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